Turbidity measurements in the sense of this invention are performed by means of a turbidity sensor especially in fresh water, water for industrial use and waste water as well as gases. Furthermore, the invention relates to measurements of similar process variables, such as solids content, and sludge, or mud, level. Measuring devices suitable for determining the corresponding process variables are produced and sold by the group of firms, Endress+Hauser, in a large number of variants, for example, under the designation “Turbimax CUS51D” and “Turbimax W CUS65”.
Usually, the sensors are arranged in a sensor body, and the determination of the process variable occurs optically. In such case, electromagnetic waves of a certain wavelength are sent from at least one transmitting unit, scattered by the medium to be measured and received by a receiving unit. The wavelengths of the electromagnetic waves of the optical components lie typically in the near infrared, for example, at 860 nm. Especially in the American market, however, also white light sources are used.
The transmitters are, most often, narrow band radiators, e.g. a light-emitting diode (LED). In such case, the LED is used for producing light lying in a suitable wavelength range. Applied as a receiver can be a corresponding photodiode, which produces from the received light a receiver signal, for example, a photocurrent or a photovoltage.
In order to make turbidity measurements comparable, turbidity standard liquids are applied, such as, for example, formazine. The most common turbidity units are FAU (Formazine Attenuation Unit), FNU (Formazine Nephelometric Unit), FTU (Formazine Turbidity Unit), NTU (Nephelometric Turbidity Unit), TU/F (Turbidity Unit/Formazine), EBC (European Brewery Convention) and ASBC (American Society of Brewing Chemists).
As a rule, turbidity sensors are calibrated with the standard, formazine.
Formazine is a mixture of hydrazine sulfate and urotropine. Production of formazine must be done very carefully, in order to achieve reproducible results.
Formazine has only a very low storability. Especially in the case of low turbidity values, the storability amounts to only a few hours.
Moreover, formazine may be mutagenic and carcinogenic.
An alternative to formazine is a reference material in the form of a solid body. Solid body standards can, in contrast, be stored for longer periods of time, to the extent that materials, which age, are avoided. They are additionally less questionable as regards handling.
Solid body standards have, until now, been made either of materials, which scatter incident light (e.g. turbid synthetic materials) or of materials, in the case of which synthetically introduced structures are responsible for the light scattering. The latter have, on the one hand, the advantage that specific, different turbidity values can be implemented and, on the other hand, that the provided structures can be limited locally to certain regions of the solid. This has until now been implemented exclusively by means of a glass body, in the case of which small foreign particles with an index of refraction different from the glass are introduced by doping. This method is like scattering the light on suspended particles in liquids and is thus suitable as a secondary standard. The process of doping the glass is, however, relatively complicated and, thus, very expensive.